Analog track and hold circuits are used in a wide range of applications such as data acquisition systems, memory systems, and oscillators. Such circuits are used to convert a changing analog voltage into a constant voltage over a predefined gating-time interval. The constant output of the circuit may then be converted, for example, to a digital signal by means of an analog-to-digital converter for various digital applications.
A track and hold circuit, or a sample and hold circuit, in its simplest form is a switch in series with a capacitor. During the time when an input voltage to the circuit is to be tracked, the switch is closed and the capacitor charges or discharges as necessary to follow the input. At the end of the tracking interval, when it is desired to hold the input voltage, the switch is opened. The capacitor is thereby prevented from charging or discharging and stores the last instantaneous value of the input voltage prior to the opening of the switch.
Modern analog track and hold circuits use very sophisticated switches, but still store the input signal as a voltage on a holding capacitor. The switch element is usually a bipolar or field effect transistor. The switch control signal is connected to the base or gate of the transistor. To track an input signal, the transistor is switched on and the voltage across the capacitor is free to follow the input voltage. To hold the voltage, the transistor is switched to the off or high-impedance state, thereby preventing the capacitor from charging or discharging.
A common problem with all analog track and hold circuits is that there are frequently unwanted voltage changes across the holding capacitor as the circuit changes between the track mode and the hold mode. These voltage changes are frequently quite fast and are referred to as "glitches." The voltage glitches result from the unwanted charging or discharging of the capacitor as the transistor switch opens and closes.
These narrow, short-duration glitches are extremely undesirable at the output of such circuits because they produce a change in the analog voltage stored on the capacitor. This problem is especially acute when the circuit is connected to a digital-to-analog converter. As the converter produces a digital equivalent of the voltage on the capacitor, the glitch will cause any digital computations subsequently performed on the digitized analog voltage to be in error.
The unwanted charging or discharging of the capacitor occurs because the transistors which do the switching are not perfect switch elements. That is, instead of making a transition from no resistance to infinite resistance, the transistor switches change from a few tens of ohms of resistance to hundreds of millions of ohms of resistance as the circuit changes between the tracking of the input analog signals to the holding of the signal. Furthermore, this resistance transition requires a finite time, as opposed to an ideal switch which switches in zero time. This imperfect action of the switch allows the capacitor to charge or discharge, thereby producing the glitches. Furthermore, as the transistor is turned off, stray capacitance can couple charge to the holding capacitor. Therefore, to the extent that the electronic switching element in a track and hold or sample and hold circuit can be made to operate more ideally and as unwanted charge coupling can be eliminated, the possibility of having voltage glitches across the holding capacitor is greatly reduced.
Accordingly, it is the principal object of the present invention to reduce the voltage glitches across the holding capacitor in analog track and hold or sample and hold circuits as these circuits change between the tracking or sampling of an input analog signal to the holding of the signal.
Another object of the present invention to provide a relatively inexpensive low-glitch current switch.
Still another object of this invention is to provide a low-glitch current switch in a manner which can be easily fabricated in integrated circuit form.
Other objects of the invention are to provide a low-glitch current switch which switches quickly, which accurately tracks an input analog voltage, and which incorporates good matching of the components to prevent or cancel unwanted charge coupling.